For logistical and security reasons there is an increasing interest for transport operators to equip vehicle trailers and semi-trailers with tracking units. These units typically comprise a GPS (Global Positioning Satellite system) receiver to provide location information and other vehicle parameter sensors and may be passive systems which log data for later retrieval, or may be active systems which include a cellular or satellite transmitter/receiver for transmission of data to a monitoring station for continual monitoring of the vehicle.
The transmitters can be programmed to provide location data at desired time intervals or may be programmed to react to particular events such as coupling and decoupling tractor units to a trailer, or opening and closing trailer doors.
Trailer units typically have no source of electrical power and when attached to a tractor the trailer electrical system is connected into the tractor electrical system which supplies a 24v dc electricity to the trailer through ISO standard connectors. When tracking units are fitted to trailers they often have their own 12v battery and a battery charger unit which can recharge the battery from the trailer electrical system during the time period that the tractor and trailer are interconnected.
It is known to use the voltage from the tractor electrical supply (24v) as a signal to monitor the operation status of the trailer i.e coupled or uncoupled in relation to a tractor.
However problems can arise due to the incorrect or incomplete coupling between the tractor and trailer so that for example the trailer electrical system did not receive continuous power but was for example coupled to the power supply only when the brakes are applied or indicators used. Even when correctly connected a continuous power supply may be interrupted due to loose contacts when driving over rough roads.
The tracking unit may operate in response to other vehicle sensed parameters and on-board sensors may be triggered by operational conditions, for example door condition sensors may be triggered by driving over bumpy roads, or by shocks caused by fork lift trucks during loading or unloading and the information transmitted may give a false indication of the trailer status.
The present inventions seeks to ameliorate the above problems and reduce the number of vehicle condition spurious signals transmitted by a tracking unit.